Processing for preparing salts of dialkylphosphinic acids

ABSTRACT

The invention relates to a process for preparing salts of dialkylphosphinic acids, which comprises 
     a) reacting alkylphosphonous and/or hypophosphorous acid and/or alkali metal salts thereof with olefins in the presence of a free-radical initiator to give dialkylphosphinic acids and/or alkali metal salts thereof and 
     b) reacting the dialkylphosphinic acids and/or alkali metal salts thereof obtained according to a) with metal compounds of Mg, Ca, Al, Sb, Sn, Ge, Ti, Zn, Fe, Zr, Ce, Bi, Sr, Mn, Li, Na and/or K to give the metal dialkylphosphinate salts. The invention likewise relates to the use of the metal dialkylphosphinate salts prepared by the process according to the invention for preparing flame retardants.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/198,540,filed Nov. 24, 1998, which is herein incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a process for preparing salts ofdialkylphosphinic acids and to the use of the dialkylphosphinate saltsprepared by this process.

BACKGROUND OF THE INVENTION

Aluminum salts of organic phosphorus-containing acids are known as flameretardants. They can be prepared by various processes.

EP-A-0 299 922 describes a process for preparing aluminum salts ofphosphoric and phosphonic esters by reacting aluminum with phosphoricacid, phosphonic acid or an ester thereof.

In the process described in EP-A-0 245 207, aluminum compounds arereacted with alkylphosphonic diesters to give the corresponding aluminumsalt.

According to EP-A-0 327 496, the reaction of aluminum hydroxide withalkylphosphonic diesters in the absence of water at approximately 180°C. likewise leads to aluminum salts of phosphnic half-esters.

EP-A-0 699 708 describes flame-retardant polyester molding compounds,the polyesters being given a flame-retardant finish by adding calciumsalts or aluminum salts of phosphinic or diphosphinic acids. Theabove-mentioned salts are obtained by reacting the correspondingdialkylphosphinic acids with calcium hydroxide or aluminum hydroxide.

DE 24 47 727 describes low-flammability polyamide molding compoundswhich comprise a salt of a phosphinic acid or of a diphosphinic acid.

However, the abovementioned processes have the disadvantage that thesuitable organic phosphorus compounds must first be prepared in alaborious manner. This applies, in particular, to the dialkylphosphinicacids, whose aluminum salts give the best results in the application asflame retardants, and for which, likewise, some synthetic pathways aredescribed.

Thus DE 21 00 779 A1 describes a process for preparing alkyldialkylphosphinates by addition of olefins having from 2 to 22 carbonatoms to alkylphosphonous esters.

In this case also, there has been the lack to date of an economicsynthesis method which leads to homogeneous products in a high yield.

SUMMARY OF THE INVENTION

The object therefore underlying the invention is to provide a processfor preparing salts of dialkylphosphinic acids in which, in aparticularly simple and economical manner, not only thedialkylphosphinic acids and/or their alkali metal salts, but also thedesired end products, that is to say dialkylphosphinic salts of certainmetals, may be prepared.

This object is achieved by a process of the type described at theoutset, which comprises

a) reacting alkylphosphonous and/or hypophosphorous acid and/or alkalimetal salts thereof with olefins in the presence of a free-radicalinitiator to give dialkylphosphinic acids and/or alkali metal saltsthereof and

b) reacting the dialkylphosphinic acids and/or alkali metal saltsthereof obtained according to a) with metal compounds of Mg, Ca, Al, Sb,Sn, Ge, Ti, Zn, Fe, Zr, Ce, Bi, Sr, Mn, Li, Na and/or K to give themetal dialkylphosphinate salts.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferably, as free-radical initiator, use is made of azo compounds.

Preferably, the azo compounds are cationic and/or non-cationic azocompounds.

Preferably, as cationic azo compounds, use is made of2,2′-azobis(2-amidinopropane)dihydrochloride or2,2′-azobis(N,N′-dimethyleneisobutyramidine)dihydrochloride.

Preferably, as non-cationic azo compounds, use is made ofazobis(isobutyronitrile), 4,4′-azobis(4-cyanopentanoic acid) or2,2′-azobis(2-methylbutyronitrile).

Preferably, as free-radical initiator, use is made of inorganic peroxidefree-radical initiators and/or organic peroxide free-radical initiators.

Preferably, as inorganic peroxide free-radical initiator, use is made ofhydrogen peroxide, ammonium peroxodisulfate and/or potassiumperoxodisulfate.

Preferably, as organic peroxide free-radical initiators, use is made ofdibenzoyl peroxide, di-tert-butyl peroxide and/or peracetic acid.

A wide selection of suitable free-radical initiators is found, forexample, in Houben-Weyl, Supplementary Volume 20, in the chapter“Polymerisation durch radikalische Initiierung” [Polymerization byFree-Radical Initiation] on pages 15-74.

Preferably, the metal compounds are metal oxides, metal hydroxides,metal hydroxide oxides, metal sulfates, metal acetates, metal nitrates,metal chlorides and/or metal alkoxides.

Particularly preferably, the metal compounds are aluminum hydroxide oraluminum sulfates.

Preferably,

a) alkali metal salts of hypophosphorous acid are reacted with olefinsin the presence of a cationic free-radical initiator to give the alkalimetal dialkylphosphinates and

b) the alkali metal dialkylphosphinates obtained according to a) arereacted with aluminum compounds to give the aluminum dialkylphosphinatesalts.

Instead of the alkali metal salts of hypophosphorous acid, an aqueoussolution of the free acid can likewise be used without problems.

Preferably, the product mixture obtained according to step a) is reactedwith the metal compounds without further purification.

In a further embodiment of the process, the product mixture obtainedaccording to step a) is worked up and thereafter only thedialkylphosphinic acids and/or alkali metal salts thereof obtainedaccording to step a) are reacted with the metal compounds.

Preferably, as olefins, use is made of unbranched or branched α-olefins.

Preferably, as olefins, use is made of those having an internal doublebond, cyclic or open-chain dienes and/or polyenes having from 2 to 20carbon atoms.

Preferably, as olefins, use is made of ethylene, n-propylene,isopropylene, n-butene, isobutene, n-pentene, isopentene, n-hexene,isohexene, n-octene, isooctene, 1-decene, 1,5-cyclooctadiene,1,3-cyclopentadiene, dicyclopentadiene and/or 2,4,4-trimethylpenteneisomer mixture.

Preferably, the olefins bear a functional group.

Suitable olefins are compounds of the formula

where R¹-R⁴ can be identical or different and are hydrogen, an alkylgroup having from 1 to 18 carbon atoms, phenyl, benzyl oralkyl-substituted aromatics.

Suitable olefins are likewise cycloolefins of the formula

in particular cyclopentene, cyclohexene, cyclooctene and cyclodecene.

Use can also be made of open-chain dienes of the formula

where R⁵-R¹⁰ are identical or different and are hydrogen or a C₁-C₆alkyl group and R¹¹ is (CH₂)_(n) where n=0 to 6. Preference is given inthis case to butadiene, isoprene and 1,5-hexadiene.

Preferred cyclodienes are 1,3-cyclopentadiene, dicyclopentadiene and1,5-cyclooctadiene, and also norbornadiene.

Preferably, the alkylphosphonous acid and/or alkali metal salts thereofare methylphosphonous acid or methylphosphonous acid and/or alkali metalsalts thereof.

Preferably, the reaction in step a) is carried out at a temperature offrom 40 to 130° C.

Particularly preferably, the reaction in step a) is carried out at atemperature of from 70 to 110° C.

Preferably, the reaction in step b) is carried out at a temperature offrom 20 to 150° C.

Particularly preferably, the reaction in step b) is carried out at atemperature of from 80 to 120° C.

Preferably, the reactions in step a) and in step b) are carried out inan acetic acid medium.

In step b), preference is also given to reaction in aqueous medium.

In this case, the reaction in step b) is carried out after adjusting toa pH range for the salt precipitation which is optimum for therespective system of dialkylphosphinic acid/metal compound.

The present invention also relates in particular to a process in whichsodium hypophosphite is reacted with ethylene in the presence of acationic or non-cationic free-radical initiator or in the presence of aperoxide free-radical initiator to give the sodium salt ofdiethylphosphinic acid as main product.

This product is then reacted according to the invention with aluminumhydroxide or an aluminum sulfate to give the aluminum salt ofdiethylphosphinic acid.

The invention also relates to the use of the metal, dialkylphosphinatesalts prepared by the process according to the invention for preparingflame retardants.

In particular, the invention relates to the use of the metaldialkylphosphinate salts prepared according to the invention forpreparing flame retardants for thermoplastic polymers such aspoly(ethylene terephthalate), poly(butylene terephthalate), polystyreneor polyamide and for thermosetting plastics.

Finally, the invention also relates to the use of metaldialkylphosphinate salts prepared by the process according to theinvention as additives in polymeric molding compounds.

The invention is described in more detail by the examples below.

EXAMPLE 1

a) Preparation of Methylethylphosphinic Acid

1000 g (12.5 mol) of methylphosphonous acid were introduced into anautoclave together with 50 g (0.18 mol, 1.5 mol %) of2,2′-azobis(2-amidinopropane)dihydrochloride and the mixture was firstheated to 60° C. with stirring. Thereafter, ethylene was introduced intothe reactor up to saturation at a pressure of 20 bar. After a reactiontime of 17 h at a maximum of 81° C., the reactor was depressurized andcooled. The yield was 1.35 kg.

³¹P-NMR analysis: Methylethylphosphinic acid: 92.4 mol % Methylbutylphosphinic acid: 6.2 mol % Methylphophonous acid: 0.9 mol %Unknown components: 0.5 mol %

b) Preparation of the Aluminum Salts

1100 g of the mixture obtained according to a) predominantly comprisingmethylethylphosphinic acid and methylbutylphosphinic acid were dissolvedin 2800 ml of acetic acid and 270 g (3.4 mol) of aluminum hydroxide wereadded. The mixture was heated for 5 hours under reflux, then cooled,filtered off by suction and dried in a vacuum drying cabinet at 135° C.In total, 1172 g of product were obtained, corresponding to a yield of97%. The content of aluminum methylethylphosphinate was 93.2 mol % andof aluminum methylbutylphosphinate was 6.1 mol %.

EXAMPLE 2

a) Preparation of Diethylphosphinic Acid (as Sodium Salt)

2.2 kg (20.7 mol) of sodium hypophosphite monohydrate were dissolved in8 kg (7.62 l) of acetic acid and introduced into an enamel steel 16 ljacketed pressure reactor. After heating the reaction mixture up to 85°C., ethylene was introduced into the reactor up to saturation via areducing valve set to 5 bar. The reaction was started by adding asolution of 56 g (1 mol %) of2,2′-azobis(2-amidinopropane)dihydrochloride in 250 ml of water withconstant stirring and was controlled via the rate of addition offree-radical initiator in such a manner that the reaction temperature inthe reactor did not exceed 95° C. at a jacket temperature of 80° C. withconstant addition of ethylene at a mean pressure of about 5 bar. Themetering time was in total 3 hours. The mixture was then given apost-reaction time of a further 3 h at 85° C. The reactor wasdepressurized, cooled to room temperature and the contents wereanalyzed.

³¹P-NMR analysis: Sodium diethylphosphinate: 87.0 mol %  Sodiumethylbutylphosphinate: 11.9 mol %  Sodium monoethylphosphinate: 0.9 mol% Sodium hypophosphite: 0.1 mol % Unknown components: 0.1 mol %

The total amount of the contents was 11.7 kg. This is equivalent to anethylene uptake of 1.2 kg (100% of theory).

b) Preparation of the Aluminum Diethylphosphinate Salt

800 g of the mixture of principally sodium diethylphosphinate obtainedaccording to a) were dissolved in 2500 ml of acetic acid and then 38 g(0.48 mol) of aluminum hydroxide were added. The mixture was then heatedfor about 4 hours under reflux, cooled and filtered off. The resultingsolids were first washed with 1 liter of glacial acetic acid, then with1 liter of distilled water and finally with 500 ml of acetone, and thendried under reduced pressure at 130° C. Yield: 183 g (92% of theory).

EXAMPLE 3

a) Preparation of Diethylphosphinic Acid (Sodium Salt)

2.12 kg (20 mol) of sodium hypophosphite monohydrate were dissolved in 7kg of acetic acid and introduced into an enamel steel 16 l jacketedpressure reactor. After heating the reaction mixture to 100° C.,ethylene was introduced into the reactor up to saturation via a reducingvalve set to 5 bar. A solution of 32.8 g (1 mol %) ofazobis(isobutyronitrile) (AIBN) in 500 g of acetic acid was addeduniformly in the course of a period of 6 h with constant stirring at anethylene pressure of 5 bar and at a temperature of 100-105° C. After apost-reaction time of 1 h, depressurization of the reactor and coolingto room temperature, the contents were analysed:

³¹P-NMR: Sodium diethylphosphinate: 91.3 mol %  Sodiumbutylethylphosphinate: 7.7 mol % Sodium ethylphosphonite: 0.7 mol %Unknown components: 0.3 mol %

The ethylene uptake was 1160 g (100% of theory).

b) Preparation of the Aluminum Diethylphosphinate Salt

520 g (6.67 mol) of aluminum hydroxide were added to the solutionobtained according to a), the mixture was heated for 4 h at 80° C. andrefluxed for a further 4 h. The solids obtained were then filtered off,washed twice, each time with 2 l of acetic acid and 2 l of water oneafter the other, and dried under reduced pressure at 130° C. Yield: 2210g (85% of theory).

EXAMPLE 4

a) Preparation of Diethylphosphinic Acid

A mixture of 2.64 kg (20 mol) of a 50% strength aqueous solution ofhypophosphorous acid and 7 kg of acetic acid was introduced into anenamel steel 16 l jacketed pressure reactor. After heating the reactionmixture up to 100° C., ethylene was introduced into the reactor up tosaturation via a reducing valve set to 5 bar. A solution of 56 g (1 mol%) of 4,4′-azobis(4-cyanopentanoic acid) in 500 g of acetic acid wasadded uniformly in the course of a period of 6 h with constant stirringat an ethylene pressure of 5 bar and a temperature of 100-105° C. Aftera post-reaction time of 1 h, depressurization of the reactor and coolingto room temperature, the contents were analysed:

³¹P-NMR: Diethylphosphinic acid: 90.6 mol %  Butylethylphosphinic acid:8.4 mol % Ethylphosphonous acid: 0.8 mol % Unknown components: 0.2 mol %

The ethylene uptake was 1160 g (100% of theory).

b) Preparation of the Aluminum Diethylphosphinate Salt

The solution obtained according to a) was very largely freed from thesolvent acetic acid on a rotary evaporator and then 10 l of water wereadded. 4500 g (3.5 mol) of a 46% strength aqueous solution ofAl₂(SO₄)₃.14H₂O were added in the course of one hour. The resultingsolids were then filtered off, washed twice each time with 2 l of aceticacid and 2 l of water one after the other, and dried at 130° C. underreduced pressure. Yield: 2520 g (82% of theory).

EXAMPLE 5

a) Preparation of Dioctylphosphinic Acid (Sodium Salt)

A solution of 1.5 g (2 mol %) of 2,2′-azobis(2-methylbutyronitrile) in50 g of acetic acid were added uniformly in the course of a period of 16h at 95° C. with constant vigorous stirring to a mixture of 42.4 g (0.4mol) of sodium hypophosphite monohydrate, 134.4 g (1.2 mol) of 1-octeneand 1 kg of acetic acid in a 2 l three-neck flask fitted with stirrer,reflux condenser and metering apparatus. After a post-reaction time of 1h and cooling to room temperature, the contents were analysed:

³¹P-NMR: Dioctylphosphinic acid: 94.1 mol %  Hexadecyloctylphosphinicacid: 4.2 mol % Octylphosphonous acid: 1.1 mol % Unknown components: 0.6mol %

b) Preparation of the Aluminum Dioctylphosphinate Salt

10.4 g (0.13 mol) of aluminum hydroxide were added to the solutionobtained according to a), heated for 4 h at 80° C. and refluxed for afurther 16 h. The resulting solids were then filtered off, washed twiceeach time with 200 ml of acetic acid and 200 ml of water one after theother and dried at 130° C. under reduced pressure. Yield: 90 g (75% oftheory).

What is claimed is:
 1. A process for preparing salts ofdialkylphosphinic acids, which comprises a) reacting alkylphosphonousand/or hypophosphorous acid and/or alkali metal salts thereof witholefins in the presence of a free-radical initiator, wherein thefree-radical initiator is an azo-compound, to give dialkylphosphinicacids and/or alkali metal salts thereof and b) reacting thedialkylphosphinic acids and/or alkali metal salts thereof obtainedaccording to a) with metal compounds of Mg, Ca, Al, Sb, Sn, Ge, Ti, Zn,Fe, Zr, Ce, Bi, Sr, Mn, Li, Na and/or K to give the metaldialkylphosphinate salts.
 2. The process as claimed in claim 1, whereinthe azo compound is a cationic azo compound or a non-cationic azocompound.
 3. The process as claimed in claim 2, wherein the cationic azocompound is 2,2′-azobis(2-amidinopropane)dihydrochloride or2,2′-azobis(N,N′-dimethyleneisobutyramidine)dihydrochloride.
 4. Theprocess as claimed in claim 2, wherein the non-cationic azo compound isazobis(isobutyronitrile), 4,4′-azobis(4- cyanopentanoic acid) or2,2′-azobis(2-methylbutyronitrile).
 5. The process as claimed in claim1, wherein the metal compounds are metal oxides, metal hydroxides, metalhydroxide oxides, metal sulfates, metal acetates, metal nitrates, metalchlorides and/or metal alkoxides.
 6. The process as claimed in claim 1,wherein the metal compounds are aluminum hydroxide or aluminum sulfates.7. The process as claimed in claim 1, wherein a) alkali metal salts ofhypophosphorous acid are reacted with olefins in the presence of acationic free-radical initiator to give the alkali metaldialkylphosphinates and b) the alkali metal dialkylphosphinates obtainedaccording to a) are reacted with aluminum compounds to give the aluminumdialkylphosphinate salts.
 8. The process as claimed in claim 1, whereinthe product mixture obtained according to step a) is reacted with metalcompounds without further purification.
 9. The process as claimed inclaim 1, wherein the product mixture obtained according to step a) isworked up and thereafter only the dialkylphosphinic acids and/or alkalimetal salts thereof obtained according to step a) are reacted with themetal compounds.
 10. The process as claimed in claim 1, wherein, asolefins, use is made of unbranched or branched α-olefins.
 11. Theprocess as claimed in claim 1, wherein, as olefins, use is made of thosehaving an internal double bond, cyclic or open-chain dienes and/orpolyenes having from 4 to 10 carbon atoms.
 12. The process as claimed inclaim 1, wherein, as olefins, use is made of ethylene, n-proplyene,isopropylene, n-butene, isobutene, n-pentene, isopentene, n-hexene,isohexene, n-octene, isooctene, 1-decene, 1,5-cyclooctadiene,1,3-cyclopentadiene, dicyclopentadiene and/or 2,4,4-trimethylpenteneisomer mixture.
 13. The process as claimed in claim 1, wherein theolefins bear a functional group.
 14. The process as claimed in claim 1,wherein the alkylphosphonous acid and/or alkali metal salts thereof aremethylphosphonous acid or ethylphosphonous acid and/or alkali metalsalts thereof.
 15. The process as claimed in claim 1, wherein thereaction in step a) is carried out at a temperature of from 40 to 130°C.
 16. The process as claimed in claim 1, wherein the reaction in stepa) is carried out at a temperature of from 70 to 110° C.
 17. The processas claimed in claim 1, wherein the reaction in step b) is carried out ata temperature of from 20 to 150° C.
 18. The process as claimed in claim1, wherein the reaction in step b) is carried out at a temperature offrom 80 to 120° C.
 19. The process as claimed in claim 1, wherein thereactions in step a) and in step b) are carried out in an acetic acidmedium.